The present invention relates to rotary fluid pressure devices, and more particularly, to such devices which include gerotor displacement mechanisms utilizing low-speed, commutating valving.
In a conventional gerotor motors utilizing low-speed, commutating valve (i.e., the rotary valve element rotates at the speed of rotation of the gerotor star rather than at the orbiting speed of the star), the valving action has typically been accomplished by means of a rotary valve member and a stationary valve member, with both valve members being separate from the gerotor displacement mechanism.
In recent years, those skilled in the art have developed what may be termed a "valve-in-star" (VIS) gerotor motor, an example of which is illustrated and described in U.S. Pat. No. 4,741,681, assigned to the assignee of the present invention and incorporated herein by reference. In a VIS motor, the commutating valving action is accomplished at an interface between the orbiting and rotating gerotor star, and an adjacent, stationary valve plate, which is typically part of the motor housing, and more specifically, part of the endcap assembly.
Although "commutating" valving action is well known to those skilled in the gerotor motor art, a brief explanation will be provided herein. In a typical gerotor motor, the ring member defines a plurality N+1 of internal teeth, and the orbiting and rotating star defines a plurality N of external teeth. The stationary valve member then defines a plurality N+1 of valve passages communicating with the expanding and contracting fluid volume chambers of the gerotor, while the rotary valve member (orbiting and rotating star in the case of a VIS motor) defines a plurality N of fluid ports at high pressure ("system pressure"), and a plurality N of fluid ports at low pressure (return or exhaust). The progressive fluid communication between each of the N ports and each of the N+1 fluid passages in the stationary valve member, as the star orbits and rotates, comprises the commutating valving.
In the subject embodiment of the present invention, the stationary valve plate and the endcap member comprise two separate members, with the endcap member and the stationary valve member cooperating to define the inlet and outlet pressure regions, while the stationary valve member alone defines the N+1 valve passages communicating with the volume chambers of the gerotor. During the development of the commercial embodiment of the invention, it has been observed that the volumetric efficiency of the motor decreases as the pressure differential across the motor increases, which is to be expected, but that the decrease in volumetric efficiency is more drastic than what would normally be expected, and certainly more drastic than what is acceptable. It has been hypothesized that the drop in volumetric efficiency reflects a progressive increase in cross-port leakage, and in particular, cross-port leakage along the interface between the endcap member and the stationary valve member.